This invention relates in general to packetized voice communication systems, and more particularly to a method of generating comfort noise at a receiver in a packetized voice system during periods of transmitter silence.
A packetized voice transmission system comprises a transmitter and a receiver. The transmitter collects voice samples and groups them into packets for transmission across a network to the receiver. The transmitter performs no operations upon the data. The data itself is companded according to u-law or A-law, as defined in ITU-T specification G.711, and is transmitted continuously at a constant TDM data rate (Time Division Multiplexing).
In order to save network bandwidth, packets of samples are only transmitted if voice activity is detected in the packet (i.e. voice data is not transmitted if the packet contains silence). It is known in the art for transmitters to test each packet for silence, prior to transmission, and after a sequence of packets is detected as containing silence, then inhibiting transmission of subsequent silence packets until the next xe2x80x9cnon-silentxe2x80x9d packet is detected. The present invention is not directed at silence detection systems for transmitters, although such systems are disclosed, for example, in U.S. Pat. Nos. 5,276,765; 5,737,695; 4,167,653; 4,277,645 and 5,867,574, and as described in co-pending commonly assigned U.S. application Ser. No. 09/580,788 the contents of which are incorporated herein by reference.
A receiver in a packetized voice system receives packets of voice data from the transmitter and transmits the voice samples at a constant rate to a digital telephone. When transmission has been suppressed as a result of the voice packets containing silence, the receiver circuit must still transmit data to the telephone at the usual rate. However, rather than transmitting pure silence code (e.g. a string of zeroes), it is customary to transmit noise (e.g. white noise or coloured noise) so that a party using the telephone is aware that the communication link with the transmitter is still active.
Two approaches are known in the patent literature for the generation of comfort noise during periods of silence. U.S. Pat. No. 3,614,399 discloses the generation of white, coloured or random noise using simple hardware located at the telephone receiver. U.S. Pat. No. 5,121,349 describes a similar noise generator which includes variable amplitude control. Both prior art approaches generate noise which is not directly related to the transmitter noise.
The second known prior art approach is disclosed in U.S. Pat. No. 5,537,509; 5,630,016; 5,812,965 and 5,809,460, each of which discloses a system for analysing transmitter noise using complex numerical signal processing. When silence, or lack of voice activity, is detected at the transmitter, the silence noise is processed by the transmitter in order to extract parameters which define the amplitude, frequency and time characteristics of the noise. These parameters are then transmitted to the receiver which regenerates the silence noise from these parameters.
According to the present invention, a method is provided for generating comfort noise at a receiver which is related to the noise characteristics at the transmitter, but does not rely on sophisticated signal processing as set forth in the prior art. More particularly, the packet buffer of the receiver is chosen to be large enough to store a plurality of voice packets but small enough such that, once the transmitter has been halted the buffer is filled with silence code from the transmitter (i.e. the transmitter halts after a predetermined time following detection of silence). The receiver detects the absence of new packets as transmitter silence. A random number generator is used to randomly address locations in the buffer for outputting samples of the transmitter""s silence code until the next non-silent voice packet is received.